05-09-2012, 01:41 PM
MEMS electrostatic micropower generator for low frequency operation
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Abstract
This paper describes the analysis, simulation and testing of a microengineered motion-driven power generator, suitable for application
in sensors within or worn on the human body. Micro-generators capable of powering sensors have previously been reported, but these
have required high frequency mechanical vibrations to excite a resonant structure. However, body-driven movements are slow and irregular,
with large displacements, and hence do not effectively couple energy into such generators. The device presented here uses an
alternative, non-resonant operating mode. Analysis of this generator shows its potential for the application considered, and shows the
possibility to optimise the design for particular conditions. An experimental prototype based on a variable parallel-plate capacitor operating
in constant charge mode is described which confirms the analysis and simulation models. This prototype, when precharged to 30V,
develops an output voltage of 250V, corresponding to 0.3J per cycle. The experimental test procedure and the instrumentation are also
described.
Introduction
Several examples of motion-driven micro-generators
capable of powering microelectronic circuits have been reported
[1–7]. Some potential applications for micro-generators
are wearable, medical electronics and remote sensing.
These generators can break the finite energy restriction of
battery powered devices by harvesting energy, in the form
of motion, from their environment. The reported examples
use a mass–spring system which resonates when the frame
of the device is vibrated. The motion of the mass relative
to the frame is damped by one of several energy conversion
mechanisms, namely electromagnetic force [1,2,5], electrostatic
force [3,7,6], or piezoelectric force [4]. The different
force-movement characteristics of these conversion
mechanisms give different operating characteristics for the
generators [8]. Thus, there are velocity-damped resonant
generators (VDRGs) based on electromagnetic damping,
and Coulomb-damped resonant generators (CDRGs) based
on electrostatic damping.
Coulomb-force parametric generator
In all three generator architectures, acceleration of the
frame by the external system creates a force f on the mass.
The product of this force and the relative displacement, f ·z,
represents work done on the damper and suspension, and
the integral of f · dz over a complete cycle represents the
converted energy per cycle. In the resonant cases, the work
can be absorbed by the damping mechanism or stored in
the spring, from where it can be released to the damper at a
different part of the motion cycle. In a non-resonant device,
however, work is only done on the damper, so to maximise
the output power this damper force should be maximised
through the whole of the relative motion. In addition, the
damping force must be less than m¨y in order for the mass
to break away from the frame. Therefore, the maximum
energy can be obtained if the mass moves during the peak of
acceleration of the source motion. If this is done, the optimal
damping force will be approximately constant at just below
m¨ymax.
Practical constraints
Constant charge and constant voltage mode electrostatic
generators have been compared in [3], where the latter is
shown to provide significantly higher output. However, this
is based on the assumption that the initial and final capacitances
C1 and C2, and the maximum voltage in the system
Vm, are the limiting factors. In such a case, and assuming
C1 C2, a constant voltage device generates 1/2C1V2m
per stroke, while a constant charge device produces only
1/2C2V2m
. However, the ultimate limit is achieved by setting
the holding force just below m¨ymax as described above, so
an optimal device would have a higher starting capacitance
in the constant charge case than in the constant voltage case.
Furthermore, a higher initial capacitance is easier to achieve
for parallel plates pulled apart (which gives constant force
in the constant charge case) than for plates sliding over each
other, because in the former case a small gap can be set by
physical stops, while in the latter case it must be maintained
over the whole range of travel by the suspension. High capacitance
is also difficult to achieve for comb drives.
Electro-mechanical simulations
Due to the small quantities of charge involved for power
generation in the CFPG, it is important to accurately simulate
the electronic components. Parasitic capacitances in the
order of 10 pF in the input stage of the control electronics
connected to the moving electrode would absorb the charge
and cancel out the generating effect of the moving electrode.
Similarly, leakage currents would drain the generated energy.
Consequently, an integrated electro-mechanical system
simulation must be capable of incorporating precise models
of specifically designed semiconductor devices, in order to
accurately specify the power extraction circuitry. SPICE is
well established for this kind of task, and in addition it is
capable of modelling a system written as a set of differential
equations, and this is the approach that has been taken
to produce an integrated electro-mechanical simulation.
Test set-up and instrumentation
Fig. 6 shows the experimental setup used for evaluating
the prototype generator. The generator and associated electronics
were mounted on a shaker table, and the position of
this was continually monitored with a linear variable displacement
transducer for low frequency motion and an accelerometer
for high frequency motion. The shaker was orientated
with its axis horizontal to minimise the effects of
gravity on the proof mass. Charge was applied to the capacitor
plates by means of a variable dc voltage source connected
between the grounded bottom electrode and the precharging
contact. The moving plate was connected permanently
to a voltage probe circuit, and at the position of minimum
capacitance to a discharge circuit.
Conclusions
An analysis of the dynamics and the maximum energy
yield of three architectures of micro-power generator has
shown that a non-resonant structure is the best choice for har
vesting energy from low frequency, large amplitude movements.
An electrostatic generator using an un-sprung proof
mass with a non-linear movement has been developed and
designated a Coulomb-force parametric generator. A CFPG
prototype has been implemented using a silicon proof mass
of 0.5 g which forms the moving plate of a parallel plate
capacitor. The precharging of this capacitor (at minimum
separation) sets the frame acceleration at which the mass
breaks away and begins to do work.